For prevention of air pollution, as devices for eliminating sulfur oxides in exhaust gases, wet-type limestone-gypsum desulfurizers have been widely put into practical use. A systematic diagram of a prior art (such as Japanese Published Unexamined Patent Application No. H06-238127), which is major equipment of exhaust gas processing devices including desulfurizers, is shown in FIG. 6.
Unprocessed exhaust gas g1 containing sulfur oxides and soot and dust generated from a thermal power plant or the like passes through a desulfurizer inlet duct 2 with an inlet damper 3 from a boiler duct 1, is pressurized by a desulfurization fan 4, and is led to an absorption tower 6 of the desulfurizer through a GGH heat recovery unit 5. The unprocessed exhaust gas g1 makes, in the absorption tower 6, gas-liquid contact in a countercurrent or parallel current with a sprayed adsorption liquid, whereby the sulfur oxides in the exhaust gas are absorbed and eliminated via the surfaces of adsorption liquid droplets, and the soot and dust is physically eliminated by a collision with the liquid droplets. Here, for the boiler duct 1, a duct part to directly pass through the inlet duct 2 and an outlet duct 7 is provided, and bypass dampers 17 and 17 are arranged at this part, and the bypass dampers 17 and 17 are opened when the unprocessed gas g1 is made to bypass the absorption tower 6 and directly flow to a chimney 13.
The minute liquid droplets accompanying the exhaust gas flow pass through the desulfurizer outlet duct 7 and are eliminated by a mist eliminator 8 installed at an outlet of the absorption tower 6, and purified processed exhaust gas g2 is discharged from the chimney 13 into the atmospheric air. At this time, in order to realize an improvement in atmospheric diffusibility of the smoke exhaust and white smoke prevention and in order to realize prevention of low-temperature corrosion of the inner surfaces of the outlet duct 7 and chimney 13, the gas is raised in temperature by a GGH reheater 9, and is then discharged from the chimney 13.
Here, between the nonleak-type GGH heat recovery unit 5 and nonleak-type GGH reheater 9, a heat medium pipeline 21, an opening/closing valve 22 of this pipeline 21, and a heat medium feeding pump 23 are provided.
In the prior art, when operation of the desulfurizer is stopped and the inlet damper 3 of the inlet duct 2 and an outlet damper 10 of the outlet duct 7 are closed, since heat released from the GGH reheater 9 has no escape, the ambient air temperature of the inside of the outlet duct 7 is raised to approximately 90-150° C., and inconveniences such that a resin element of the mist eliminator 8 present at an upstream side of the GGH reheater 9 is deformed by high heat and a thermal degradation occurs on a corrosion preventing lining of the inner surface of the outlet duct 7 have existed.